X-RAY INTERACTION WITH MATTER

COLLIMATE OR NOT TO COLLIMATE?!

SCATTER

X-RAYS WITH LOW ENERGIES –APPROX. 10 KEV INTERACT WITH MATTER

THROUGH COHERENT SCATTERING

Thomson scattering (R), also known as Rayleigh, coherent, or classical scattering, occurs when the x-ray photon interacts with

the whole atom so that the photon is scattered with no change in internal energy

to the scattering atom, nor to the x-ray photon. Thomson scattering is never more than a minor contributor to the absorption coefficient. The scattering occurs without the loss of energy. Scattering is mainly in

the forward direction.

Coherent Scattering

Λ1= Λ2

μ1= μ2

APPROX. 5% OF X-RAYS UNDERGO COHERENT

SCATTERING AT 70 KVP

SCATTER

COMPTON EFFECT-INCOHERENT SCATTERING

Compton Scattering (C), also known a incoherent scattering, occurs when the incident x-ray photon ejects a electron from an atom and an x-

ray photon of lower energy is scattered from the atom. Relativistic energy and momentum are

conserved in this process and the scattered x-ray photon has less energy and therefore greater

wavelength than the incident photon. Compton Scattering is important for low atomic number

specimens. At energies of 100 keV -- 10 MeV the absorption of radiation is mainly due to the

Compton effect.

MOST LIKELY TO OCCUR WITH

OUTER SHELL ELECTRONS

ANGLES OF DEFLECTION OF PHOTONS

0-180 DEGREES

0 DEGREE DEFLECTION0 DEGREE DEFLECTION0 ENERGY TRANSFER0 ENERGY TRANSFER

BACKSCATTER

COMPTON

PROBABILITY=

1E

Energy Probability

Atomic number No effect on Probability!!

ENERGY TRANSFER=ENERGY TRANSFER=

THE ENERGY OF SCATTERED X-RAY +

THE ELECTRON BINDING ENERGY +

THE KINETIC ENERGY OF RECOIL ELECTRON

Photoelectric (PE) absorption of x-rays occurs when the x-ray photon is absorbed resulting in the

ejection of electrons from the inner shell of the atom, resulting in the ionization of the atom. Subsequently, the ionized atom returns to the

neutral state with the emission of an x-ray characteristic of the atom. This subsequent emission of lower energy photons is generally absorbed and does not contribute to (or hinder) the image making process. Photoelectron absorption is the dominant

process for x-ray absorption up to energies of about 500 KeV. Photoelectron absorption is also

dominant for atoms of high atomic numbers.

PROBABILITY=

Z3

E3

Atomic number Probability

Energy Probability

ENERGY TRANSFER

ENERGY OF INCIDENT PHOTON=ENERGY OF INCIDENT PHOTON=

THE ELECTRON BINDING ENERGY+

THE KINETIC ENERGY OF THE ELECTRON

THE PHOTOELECTRIC EFFECT IS AN X-RAY

ABSORPTION INTERACTION

PROBABILITY OF COMPTON INTERACTION AND

PHOTOELECTRIC INTERACTION IS THE SAME:

• IN SOFT TISSUE AT 20 keV

• IN BONE AT 40 keV

Compton vs Photoelectric

Compton

Photoelectric

Probability of

interaction

X-ray energy

bone

s. tissue

40 keV

20 keV

Pair Production (PP) can occur when the x-ray photon energy is greater than 1.02 MeV, when an electron and positron are created with the annihilation of the x-ray photon. Positrons are very short lived and disappear (positron annihilation) with the formation of two photons of 0.51 MeV energy. Pair production is of particular

importance when high-energy photons pass through materials of a high atomic number.

Energy: > 1.02 MeV

ANNIHILATION

PROBABILITY=

E

Energy Probability

Photodisintegration (PD) is the process by which the x-ray photon is captured by the nucleus of the atom

with the ejection of a particle from the nucleus when all the energy of the x-ray is given to the nucleus. Because of the

enormously high energies involved, this process may be neglected for the

energies of x-rays used in radiography

ENERGY > 10 MeV